行为型设计模式之状态模式(State)

结构
意图	允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。
适用性	一个对象的行为取决于它的状态, 并且它必须在运行时刻根据状态改变它的行为。 一个操作中含有庞大的多分支的条件语句，且这些分支依赖于该对象的状态。这个状态通常用一个或多个枚举常量表示。通常, 有多个操作包含这一相同的条件结构。S t a t e模式将每一个条件分支放入一个独立的类中。这使得你可以根据对象自身的情况将对象的状态作为一个对象，这一对象可以不依赖于其他对象而独立变化。

 using System;

     abstract class State
     {
         protected string strStatename;        

         abstract public void Pour();
         // do something state-specific here
     }

     class OpenedState : State
     {
         public OpenedState ()
         {
             strStatename = "Opened";
         }
         override public void Pour()
         {
             Console.WriteLine("...pouring...");
             Console.WriteLine("...pouring...");
             Console.WriteLine("...pouring...");
         }
     }

     class ClosedState : State
     {
         public ClosedState()
         {
             strStatename = "Closed";
         }
         override public void Pour()
         {
             Console.WriteLine("ERROR - bottle is closed - cannot pour");
         }
     }

     class ContextColaBottle
     {
         public enum BottleStateSetting {
             Closed,
             Opened
         };

         // If teh state classes had large amounts of instance data,
         // we could dynamically create them as needed - if this demo
         // they are tiny, so we just  create them as data members
         OpenedState openedState = new OpenedState();
         ClosedState closedState = new ClosedState();

         public ContextColaBottle ()
         {
             // Initialize to closed
             CurrentState = closedState;
         }

         private State CurrentState;

         public void SetState(BottleStateSetting newState)
         {
             if (newState == BottleStateSetting.Closed)
             {
                 CurrentState = closedState;
             }
             else
             {
                 CurrentState = openedState;
             }
         }

         public void Pour()
         {
             CurrentState.Pour();
         }
     }

       /// <summary>
     ///    Summary description for Client.
     /// </summary>
     public class Client
     {
         public static int Main(string[] args)
         {
             ContextColaBottle contextColaBottle = new ContextColaBottle();

             Console.WriteLine("initial state is closed");

             Console.WriteLine("Now trying to pour");
               contextColaBottle.Pour();

             Console.WriteLine("Open bottle");
             contextColaBottle.SetState(ContextColaBottle.BottleStateSetting.Opened);

             Console.WriteLine("Try to pour again");
             contextColaBottle.Pour();

             return ;
         }
     }

状态模式